DiscordJS cannot get _roles field after fetching member

I am fetching a member as:

...
private user: GuildMember;
...

try { 
    this.user = await interaction.guild?.members.fetch(SomeUserId)
}catch (e: unknown) {
    return await interaction.editReply("Couldn't find the specific user the in server.")
}

this.user will contain all the information regarding that specific. It also includes a field _roles:['123456789', '123456789']. How can I get this specific field? Note that doing the following results in ‘_roles does not exist on type GuildMember’:

console.log(this.user._roles)

Any way to access this field?